1. Field of the Invention
The present invention relates to a conductive paste and a ceramic substrate, and more particularly to a conductive paste used for forming a via-hole in a ceramic substrate and a ceramic substrate which is formed using the paste.
2. Background Art
Conventionally, a multi-layered ceramic substrate in which inner electrodes and circuits are connected with via-holes has generally been produced through the following method.
First, a hole for providing a via-hole is formed in a ceramic green sheet by use of a drill or a punch, and the hole is filled with a conductive paste or conductive metal powder. Then, an electrode or a circuit is formed on a surface of the ceramic green sheet by application of a conductive paste thereto by, for example, a screen printing method. Subsequently, a plurality of the thus-formed ceramic green sheets are laminated and pressed, and the resultant laminate is cut so as to form a ceramic substrate of a predetermined size. As a result of the cut piece being fired, the ceramic and the conductive paste or conductive metal powder packed in the hole for providing a via-hole are co-sintered to thereby establish electric contact between conductor circuits in the ceramic substrate. Thus, a multi-layered ceramic substrate is obtained.
In the above-described conventional method for producing a ceramic substrate, copper powder is used as a conductive material for forming a via-hole, in view of low specific resistivity, resistance to migration generation and low cost. Such copper powder is mixed and dispersed in an organic vehicle in which a resin component such as ethyl cellulose has been dissolved, to thereby serve as a conductive paste.
In the conventional method for producing a ceramic substrate, the ceramic green sheets and via-holes filled with metal powder are simultaneously sintered during the firing step. However, the method has drawbacks. When via-holes are filled with a conductive paste and the resultant ceramic is fired, the obtained metal conductor in the via-holes generates cracks and the ceramic is chipped due to either excessive or insufficient amounts of the paste in the via-holes or the difference in shrinkage between the paste and the ceramic green sheets during firing. Such cracks of the metal conductor in the via-holes and chipping of the ceramic causes poor electric conduction and structural deficiency in the via-holes to thereby decrease reliability of the obtained ceramic substrate.